This invention relates to data communications networks. In particular, this invention relates to modulator-demodulator circuits, also known as modems that are used with multipoint data communication networks. More particularly, this invention relates to multipoint modems coupled together through analog communications channels.
Computers and other types of DTE (Data Terminal Equipment) devices usually exchange digital information over telephone communication channels using so called modulator-demodulator devices or modems. Most modems receive digital information from a computer (DTE), modulate the digital information by some predetermined frequency f.sub.c, and transmit the modulated digital information in an analog form onto the telephone network. Similarly, these modems demodulate an analog signal received off of a telephone communications channel using the f.sub.c carrier signal to recover any digital information therein.
In most geographic areas the public switched telephone networks convert analog information carried between switching offices to a digital format after it leaves a local end-office. In many areas however, the telephone transmission network between switching centers is still analog. In areas where transmission facilities are analog, analog multiplexing techniques, such as frequency division multiplexing or group-band multiplexing are used to carry more information more efficiently. Frequency division multiplexing or group band multiplexing frequency shifts multiple signals, each being a relatively low bandwidth signal, each to a separate frequency band or ranges at a much higher frequency. The high-frequency signal, comprised of multiple separate information signals, each transmitted using some media such as microwave or coax cable, for example, to a receiving office which de-multiplexes the information. For example, approximately 600 conversations, each having a frequency bandwidth between 200 and 3400 Hz. can be up-shifted in frequency to occupy separate 3200 Hz. channels between 564 KHz and 3084 KHz. For more details on the actual implementation, see e.g., "Transmission Systems for Communications", Bell Telephone Laboratories Revised 4th ed. Dec. 1971, Sec. 6.2, P. 124-139.
A common problem experienced with multipoint modem networks used in geographic areas where frequency division multiplexing is used to link telephone end-offices is the central modem's loss of data caused by its inability to identify the offset carrier on frequency f.sub.c (used by the central and slave modems to exchange digital information on the analog phone lines) in a short duration message sent to the central modem by a slave modem. When phone signals are frequency multiplexed on an analog communications channel, the frequency source used at one end to up-shift the signals may be slightly different from the frequency source used at the other end to down-shift the signals. In a frequency division multiplexing network, a frequency discrepancy between a local oscillator at one end of the communications channel and a local oscillator at the other end of the communications channel can change f.sub.c impeding the reliable detection of data by a modem. Since most modems modulate a carrier frequency f.sub.c, which is typically a low frequency signal near 1000 to 2000 Hertz, and since the end-office may frequency modulate this f.sub.c -based signal by some other high-frequency signal, a small difference, or offset, in the signals used to modulate and demodulate the f.sub.c modulated modem information, may significantly affect the frequency of f.sub.c when frequency division multiplexed channels are de-mulitplexed.
In a multipoint data communications system, where a single master modem talks to a plurality of slave modems simultaneously, all of the slave modems include phase-lock loop type circuits that have the capability of detecting offset on f.sub.c 's in the analog signal sent to them. These phase-locked-loop circuits all require a finite length of time during which they attempt to lock onto an incoming signal and learn the offset on f.sub.c is. In a multipoint data communications network, most of the slave modems receive substantially long signals from the master upon which their phase-locked loops can train to reliably detect and track f.sub.c including any changes in f.sub.c due to the communications channel. In multipoint modem systems however, a master typically receives only short duration messages from the slave modems when it polls a slave modem it wishes a response from. When a short duration message sent from a slave to a central modem is slightly off frequency, the master modem receiver circuitry may miss all or part of the message from the slave modem if the receiver section of the master control modem is unable to lock onto the f.sub.c that includes an offset. (Similarly, any short duration message sent to any modem with an off-frequency f.sub.c may be all or partially lost.)
Where an analog telephone network serves multipoint modems, a data communications device, or modem, that corrects for changes in f.sub.c caused by the communications channel would be an improvement over the prior art. A modem at either the master control end or the slave end of such a communications channel that has the capability of adjusting its operating characteristics to compensate for changes in the carrier frequency that the modems would use to reduce the likelihood of lost data caused by offset f.sub.c frequencies.